[0001] The present invention relates to a process for manufacturing a high-scale integration
MOS (Metal-Oxide Semiconductor) device, and to a corresponding MOS device.
[0002] As is known, the advent of large-scale integration in the fabrication of MOS devices,
in particular, below the level of one micron (ULSI - Ultra-Large Scale Integration),
calls for a drastic reduction in the vertical and lateral dimensions of the body,
source, and drain regions (the latter in the case of lateral-conduction MOS devices),
so as to obtain shallow junctions or ultra-shallow junctions. Said size reduction
could in theory be achieved by reducing the thermal budget associated to the MOS device,
but this solution is not efficient for activation of the heavily doped shallow regions
that constitute the source and drain regions of the device (in what follows defined
as source/drain regions in the case of a lateral-conduction device, and as source
regions in the case of a vertical-conduction device).
[0003] In N-channel devices, implants are usually made with low-diffusivity dopants, such
as arsenic (As), followed by a rapid thermal annealing (RTA) treatment for dopant
activation. The RTA treatment enables, as compared to a traditional thermal treatment,
reduction of the thermal budget of the process; however, it is not efficient in the
case of high-dose implants. In particular, RTA treatment does not enable an efficient
removal of the crystallographic defects due to ion implantation, which are responsible
for degradation of the electrical performance of the body/drain junctions and/or of
the gate oxide. In P-channel devices, the high diffusivity of the dopant boron (B)
atoms, normally used for the implantation process, does not enable the formation of
shallow junctions. At the same time, alternative low-diffusivity dopants, such as
for example indium (In), are difficult to use for source/drain implants, due to the
poor properties of electrical activation.
[0004] Conventional thermal treatments, even if rapid-annealing methods are used, do not,
in any case, enable an accurate control of the dopant diffusion and are consequently
not indicated for the formation of ultra-shallow doped regions, where the repeatability
of the process is fundamental for precisely controlling characteristics such as the
threshold voltage and the channel length.
[0005] In the last few years, the use of alternative techniques has consequently been proposed,
in particular, excimer-laser irradiation techniques for local heating and possibly
melting of silicon regions to enable rapid diffusion and activation of dopant atoms.
Excimer-laser-annealing (ELA) technology enables extremely shallow doped regions (with
a depth smaller than 0.1 µm) to be obtained, with excellent electrical characteristics,
in particular, with an extremely small sheet resistance. Figure 1 shows in this regard
a graph illustrating the pattern of the sheet resistance Rs as the junction depth
varies, in the case of traditional RTA technology (dashed line) and ELA technology
(solid line), in which the boxes indicate the technological integration nodes.
[0006] In detail, excimer-laser irradiation enables melting of silicon regions having dimensions
that can be controlled with extreme precision by adjusting the irradiation energy.
During melting, the crystallographic defects due to ion implantation are completely
eliminated, and dopants diffuse rapidly within the molten region. During a subsequent
liquid-phase epitaxial process, the dopant is then incorporated in the regrowth region.
Thanks to the effective thermal dissipation towards underlying silicon regions, the
temperature remains sufficiently high for a solid-phase diffusion only for tens of
nanoseconds, a time that is not sufficient for the occurrence of a significant transportation
of dopant atoms beyond the molten region.
[0007] Even though the use of this technology in the MOS-device manufacturing process is
particularly advantageous, it involves, however, considerable problems. In fact, high
laser-annealing irradiation energies are required to completely melt the doped source/drain
regions where a large number of implant-generated crystallographic defects is present.
Consequently, there exists the risk that irradiation will cause melting and deformation
of the structures that make up the MOS device. In particular, the body and source/drain
implants are performed in a step subsequent to formation of gate structures (which
include, in a known way, at least one region of silicon oxide surmounted by a gate
electrode, normally of polycrystalline silicon), self-aligned to the gate structures
or to spacers formed laterally to the gate structures. The ELA process causes melting
and deformation, in particular, of the gate electrodes, jeopardizing the electrical
characteristics of the MOS device. It is consequently necessary to reach a compromise
in the determination of the irradiation energy, taking into account the maximum energy
that the MOS structure is able to withstand without any deformations occurring. The
problem of the high energy required is moreover aggravated by the geometrical structure
of the region to be irradiated. In this regard see Figure 2, which is a schematic
perspective cross-sectional view of a typical geometrical structure of a MOS device
1 of a known type. The MOS device 1 comprises: a substrate 2 having a top surface
2a; gate structures 3 (illustrated in an extremely simplified way), formed on the
top surface 2a of the substrate 2, which succeed one another in a first direction
x and extend, parallel to one another, in a second direction y substantially perpendicular
to the first direction x; and source/drain regions (not illustrated) constituted by
doped regions formed in surface portions of the substrate 2 adjacent to, and partially
underlying, the gate structures 3. The source/drain regions are formed within source/drain
windows 5, defined by contiguous gate structures 3 arranged in the first direction
x. The source/drain windows 5 have the shape of narrow channels, for example with
a width 1 of 600 nm and a height h of 700 nm. The laser irradiation for annealing
of the source/drain regions and activation of the respective dopants occurs in a direction
normal to the top surface 2a of the substrate 2, within the source/drain windows 5.
On account of the wavelike nature of the laser radiation and of the reduced dimensions
of the regions to be irradiated, phenomena of optical interference with the side walls
of the gate structures 3 occur, with the result that peripheral portions of the source/drain
regions in the proximity of said walls receive a lower intensity of irradiation than
do the central portions. The thermal difference caused by said phenomena of interference
is moreover accentuated by phenomena of thermal dissipation from the aforesaid peripheral
portions towards adjacent cold regions of the substrate 2, situated underneath the
gate structures 3. The result is a non-uniform heating of the source/drain regions,
with the temperature T varying according to the pattern illustrated in the detail
of Figure 2 within the source/drain windows 5, with a maximum at the central portions
of the source/drain regions, and a minimum at the peripheral portions thereof. In
order to obtain a sufficient annealing of the peripheral portions of the source/drain
regions, with a complete removal of the crystallographic defects, it is consequently
necessary to use a high energy density of irradiation, even higher than 1 J/cm
2, which causes, however, deformations of the MOS-device structure.
[0008] By way of example, Figures 3a and 3b show photographs taken with a transmission electron
microscope (TEM) of a section of a portion of the MOS device 1. In detail, Figure
3a corresponds to an irradiation at an energy such as not to remove completely the
defects (which are in fact highlighted in the areas circled in black in Figure 3a).
In this case, minimal deformations appear in the MOS device. Instead, Figure 3b corresponds
to the use of a high irradiation energy, which indeed leads to complete elimination
of the implantation defects, but also to a corresponding evident deformation of the
gate structures (in particular, a lateral leakage of the gate electrode polysilicon
occurs).
[0009] In an attempt to overcome the problems described above, it has been proposed, for
example in
US 5,401,666, to exploit the modulation effect in the radiation transmission by layers of silicon
oxide with different thicknesses. In particular, it has been proposed to coat the
gate structures at the top, prior to carrying out laser irradiation, with a silicon-oxide
layer having a thickness such as to have a maximum reflectivity to the laser radiation
at the frequency used, in order to reduce the intensity of the laser radiation absorbed
by the underlying gate electrode, and consequently limit its deformation; in addition,
it has been proposed to coat the source/drain regions, where it is intended to activate
the dopant, with a silicon-oxide layer of a thickness such as to have minimum reflectivity
to the laser beam, and thus maximize the intensity of the laser radiation absorbed
by the underlying regions. However, at the high levels of energy involved, also this
solution is not sufficient to prevent deformation of the gate structures. Therefore,
in the manufacturing processes of MOS transistor devices using ELA techniques, currently
two distinct steps are envisaged for the implantation and activation of dopants in
the source/drain regions.
[0010] In detail (Figure 4a), after the formation of the gate structures 3 (illustrated
schematically) on the substrate 2, a low-dose ion implantation is performed within
the source/drain windows 5 to produce lightly doped source/drain regions 6, which
are then activated via a thermal process of a traditional type, for example an RTA
process. In a subsequent step (Figure 4b), a layer of dielectric material, for example
silicon oxide, is deposited, and subsequently etched in an anisotropic way (for example,
by means of plasma etching), for the formation of spacers 7 at the sides of the gate
structures 3 and of protection regions 8 on the gate structures 3. Then, a second
ion implantation, at a high dose, is performed within the source/drain windows 5 in
a way self-aligned to the spacers 7 to produce heavily doped source/drain regions
9. Next, the heavily doped source/drain regions 9 are subjected to an ELA process
(indicated by the arrows) for activation and diffusion of the dopants. At the end
of this process, the source/drain regions of the MOS transistor device comprise two
distinct regions with different levels of doping and different vertical diffusion
and having a different sheet resistance, namely: the lightly doped source regions
6, situated partially underneath the gate structures 3, which form, together with
the body regions (not illustrated), the channel of the MOS transistor device; and
the heavily doped source/drain regions 9, adjacent to the lightly doped source/drain
regions 6, which act as low-resistivity connections with corresponding contact metallizations
(not illustrated). With the process described, the regions with a higher concentration
of dopant (and hence with a greater concentration of defects) are confined, by means
of the spacers, far from the channel region or from the gate region of the MOS device,
and so do not limit the quality and the characteristics of the device (in fact, in
a known way, said defects are harmful if located in an area corresponding to the gate
or channel regions). The energy of the ELA irradiation can thus be the minimum necessary
for melting the silicon and activating the dopants, without excessive concern for
annealing of the crystallographic defects.
[0011] The manufacturing process described previously can be used for lateral-diffusion
MOS devices, but is not, however, optimized for vertical-diffusion MOS devices (VDMOS),
such as power MOS transistors or IGBTs. In this case, the presence of a single high-doping
source region is advantageous for optimization of the on-resistance and of the forward
transconductance of the devices. With the process described, two source regions are
obtained, one of which (in an area corresponding to the channel region) has a higher
sheet resistance. Furthermore, in the VDMOS, the threshold voltage is heavily dependent
upon the lateral diffusion of the source regions so that it is necessary to carry
out an accurate control of said diffusion in order to obtain a good repeatability
of the threshold voltage value (which is something that does not occur with the process
described, in which the lateral diffusion is controlled, with low precision, by means
of a conventional thermal process). It would then be advantageous to use a process
of laser irradiation, controllable in an accurate way, to produce a single heavily
doped source region having a low sheet resistance and without any residual crystallographic
defects.
[0012] The aim of the present invention is consequently to provide a process for manufacturing
a MOS device that will enable the aforementioned disadvantages and problems to be
overcome and, in particular, that will enable exploitation of the advantages of a
laser irradiation, so achieving a complete annealing of the implantation defects,
and at the same time overcoming the problems due to the high energies necessary for
the irradiation and the consequent risks of deformation of the MOS device structure.
[0013] According to the present invention, a process for manufacturing a MOS device and
a MOS device are consequently provided, as defined in claims 1 and 20, respectively.
[0014] For a better understanding of the present invention, preferred embodiments are now
described, purely by way of non-limiting example and with reference to the attached
drawings, wherein:
- Figure 1 shows a plot of the sheet resistance as a function of a junction depth, for
different annealing techniques;
- Figure 2 is a schematic perspective cross-sectional view of the geometrical structure
of a MOS device of a traditional type, which undergoes laser-irradiation annealing;
- Figures 3a-3b show TEM cross-sectional photographs of a portion of a MOS device subjected
to laser irradiation at different energies;
- Figures 4a and 4b show successive steps of a process for manufacturing a MOS device
of a known type;
- Figure 5a is a cross-sectional view of a dielectric mirror, in particular, a Bragg
reflector;
- Figure 5b is a table indicating the value of the refractive index in the visible of
some materials that can be used for producing the dielectric mirror of Figure 5a;
- Figures 6-11 show sections through a wafer of semiconductor material corresponding
to successive steps of a process for manufacturing a MOS device, according to a first
embodiment of the present invention;
- Figures 12a, 12b show variants of an initial step of a process for manufacturing a
MOS device, in accordance with a second embodiment of the present invention;
- Figures 13-18 show subsequent steps of the process in accordance with the second embodiment
of the present invention; and
- Figures 19-23 show sections through a wafer of semiconductor material corresponding
to successive steps of a process for manufacturing a MOS device, according to a third
embodiment of the present invention.
[0015] The general idea underlying the present invention is to use, in the manufacturing
process of a MOS device, a dielectric mirror appropriately arranged and configured
so as to reflect much of the incident laser radiation during a step of laser annealing
so as to prevent damage to the MOS device structure. As will be described in detail
hereinafter, the dielectric mirror also performs an active function of electrical
insulation within the resulting MOS device.
[0016] As is known, dielectric mirrors are used in numerous optical applications, for example
in the fields of laser, astronomy, and medicine, to obtain high levels of reflectivity
with low losses, if compared to metal mirrors, thanks to the low absorption of the
materials used. In detail, a dielectric mirror comprises a plurality of superimposed
thin layers made of (usually two) different materials having different refractive
indices. The reflections that are generated at the interface between the various layers,
in a certain range of wavelengths, interfere constructively, generating a high overall
reflectivity of the dielectric mirror (even higher than 99%). In particular, one aspect
of the present invention envisages the use of a type of dielectric mirror known as
the "Bragg reflector". As is known, and as illustrated in Figure 5a, a Bragg reflector
10 is constituted by a sequence of identical pairs of alternating and superimposed
layers made of two different materials, designated by 10a and 10b, the first having
a high refractive index n
h and the second a low refractive index n
l. The optical thickness of said layers, designated, respectively, by n
h·d
h and n
l·d
l (d
h and d
l being the thicknesses of the two layers) is equal to a multiple n of one quarter
of the wavelength λ
0 at which it is intended to maximize the reflectivity; i.e., it is equal to n·λ
0/4 (in the case in point, λ
0 is the frequency of the laser with which a subsequent ELA treatment is carried out).
By way of example, given in the table of Figure 5b is the value of the refractive
index in the visible of some materials that can be used for producing the dielectric
mirror. The reflectivity R of a Bragg reflector can be evaluated to a first approximation
using the following relation:
where n
s is the refractive index of a substrate (for example, silicon) on which the dielectric
mirror is formed, and N is the number of pairs of layers with low and high refractive
index. It is possible to carry out accurate calculations on the reflectivity R, and
on the dielectric mirror in general, via purposely provided programs for design of
dielectric mirrors, such as Optilayer (www.optilayer.com) and TFCALC (www.sspectra.com),
which enable, for example, ready evaluation of the reflectivity R at various wavelengths.
[0017] A process for manufacturing a high-scale integration MOS device according to a first
embodiment of the present invention is now described.
[0018] In detail (Figure 6), in a first step of the manufacturing process, active areas
(not illustrated) are defined (in a known manner which is not described in detail)
in a layer of semiconductor material 20, for example of an N type. In the active areas,
MOS transistors will be formed. Next, on the layer of semiconductor material 20 are
formed in succession: a thin gate-oxide layer 21, for example with a thickness of
10-100 nm; and an appropriately doped polysilicon layer 22, which will form the gate
electrodes of the MOS transistors and has a thickness, for example, of 150-600 nm.
Advantageously, a layer of metallic material 23 can be formed (for example, by deposition)
on the polysilicon layer 22 in order to reduce the resistance of the gate electrodes
of the MOS transistors. The layer of metallic material 23 can be constituted by a
metal (for example W, Mo, Ti, Ta, etc.), or else by a compound formed by silicon and
a metal (for example, CoSi
x, PtSi
x, TiSi
x, WSi
x, etc.). Next, according to an aspect of the present invention, a dielectric mirror
10 is formed on the layer of metallic material 23; the dielectric mirror 10 is configured
to reflect an incident laser radiation during a subsequent step of laser annealing.
In detail, the dielectric mirror 10 comprises, as previously indicated, a succession
of identical pairs of a first mirror layer 10a and a second mirror layer 10b superimposed
on one another, the number N of said pairs being a function of a desired reflectivity
R and of a maximum thickness of the dielectric mirror 10, which must be compatible
with the MOS structure (in particular, it must be less than 1-2 µm). The first mirror
layer 10a and second mirror layer 10b have refractive indices that are different from
one another (in particular, respectively, a high refractive index n
h and a low refractive index n
l) and are made of materials that can undergo deposition with layers of controlled
thickness on the layer of semiconductor material 20, and that have a melting point
higher than the one that can be reached during a subsequent process of laser annealing.
The difference in refractive index between the two layers must be such that the thicknesses
involved are compatible with a maximum thickness of the MOS structure resulting from
the process. Furthermore, preferably, at least one between the first mirror layer
10a and the second mirror layer 10b is constituted by an insulating material in such
a manner that the dielectric mirror 10 will have an insulating function within the
MOS structure. According to a particular aspect of the present invention, the dielectric
mirror 10 is a Bragg reflector; namely, the optical thickness of the first mirror
layer 10a and of the second mirror layer 10b is equal to a multiple of a quarter of
the wavelength λ
0 of the excimer laser that will be used for the subsequent annealing step (in particular
λ
0 is 308 nm when the laser used is an XeCl laser). The materials used in producing
the dielectric mirror must not be absorbent to the laser radiation. For example, the
first mirror layer 10a can be constituted by silicon oxide SiO
2 and have a thickness equal to 52 nm (n(SiO
2) = 1.485 at 308 nm), and the second mirror layer 10b can be constituted by silicon
nitride Si
3N
4 and have a thickness equal to 38 nm (n(Si
3N
4) = 2 at 308 nm). Furthermore, the number N of pairs of layers constituting the dielectric
mirror 10 is, for example, less than five, advantageously equal to three. A buffer
layer 24 is then formed (for example, by deposition) on the dielectric mirror 10;
also the buffer layer 24 is not absorbent to laser radiation and preferably comprises
one of the two materials of which the dielectric mirror 10 is made, and has an optical
thickness equal to n·λ
0/2-n
cd
c, where n
c and d
c are, respectively, the refractive index and the thickness of a capping layer that
will be subsequently formed on the dielectric mirror 10. Furthermore, the buffer layer
24 must be readily removable with etching processes compatible with the remaining
structure. The buffer layer 24 can be formed sequentially after the formation of the
dielectric mirror 10 and, in the case where the buffer layer 24 is made of the same
material constituting the last layer of the dielectric mirror 10, it will simply be
a matter of growing as final layer of the dielectric mirror, a layer having a total
optical thickness equal to X
0/4+n·λ
0/2-n
cd
c.
[0019] Next (Figure 7), through a suitable mask, a selective anisotropic etch (dry etch)
is carried out of the buffer layer 24, of the superimposed layers constituting the
dielectric mirror 10, of the metallic layer 23 (if present), and of the polysilicon
layer 22 so as to define: gate structures 26 (illustrated by way of example in Figure
7 is just one of said gate structures 26) of the MOS device, which are surmounted
by the dielectric mirror 10; and source/drain windows 27 set between adjacent gate
structures 26 in an area corresponding to which ion implants of source/drain regions
of the MOS device will be performed. A first ion implant is then performed within
the source/drain windows 27, for example of a P type with boron atoms (B), with a
dose of 10
13-10
14 atoms/cm
2 and an energy of between 20 keV and 150 keV, in order to form body wells 28 in a
first surface portion 20a of the layer of semiconductor material 20, in a position
adjacent to, and partially underneath, the gate structures 26. The previously implanted
dopant atoms are activated with a conventional thermal process, for example having
a duration of 30 minutes at a temperature of 900-1050°C. In particular, the thermal
process is carried out in an oxidizing environment so as to cause an oxidation of
side walls of the polysilicon electrode 22 and the formation of oxide regions 29 at
the sides thereof. Then, after regions of the layer of semiconductor material 20 that
are to undergo a source/drain implant have possibly been defined via an appropriate
masking, a second ion implantation is performed, at a high dose, in the example of
an N type with atoms of arsenic (As) or phosphorus (P) with a dose of 10
14-10
15 atoms/cm
2 and an energy of between 20 keV and 80 keV, in order to form source/drain regions
30 within the body wells 28, in a position adjacent to the gate structures 26.
[0020] Next (Figure 8), a capping layer 32 is deposited on the structures previously formed;
the capping layer 32 is conformable and not absorbent to the laser radiation, and
has the functions of improving the energy coupling with the laser beam of the implanted
source/drain areas in the subsequent step of annealing and of containing any possible
lateral deformations of the gate structure. As anticipated, the capping layer 32 has
a thickness d
c and a refractive index n
c, and the thickness of the buffer layer 24 previously defined is equal to n·λ
0/2-n
cd
c. Consequently, the combined presence of the capping layer 32 and of the buffer layer
24 does not alter the optical properties of the dielectric mirror 10, and, in particular,
does not reduce the reflectivity thereof. In fact, the total optical thickness overlying
the dielectric mirror 10 is equal to n·λ
0/2, and, as is known, optical thicknesses of n·λ
0/2 are neutral to a radiation having a wavelength λ
0. For example, in the case where the buffer layer 24 and the capping layer 32 are
both made of silicon oxide, by choosing a thickness d
c equal to 47 nm two effects can be obtained simultaneously: that of minimizing the
reflectivity of the surface of the source/drain areas at the wavelength of 308 nm,
and that of leaving the reflectivity of the dielectric mirror 10 unaltered. Likewise,
in the case where both the buffer layer and the capping layer are made of silicon
nitride, it is advantageous to choose a thickness d
c of the capping layer 32 equal to 33 nm to minimize the reflectivity of the source/drain
areas at the wavelength of 308 nm, and a thickness of the buffer layer 24 equal to
43 nm to maintain the reflectivity of the dielectric mirror 10 unaltered. In general,
the capping and buffer layers can also be made of materials that are different from
one another, and in turn different from the ones used in producing the dielectric
mirror 10, albeit guaranteeing the required dual function of improving the energy
coupling with the laser beam and of not appreciably reducing the reflectivity of the
dielectric mirror. The advantageous configuration is that whereby the capping layer
32 is made of silicon nitride with a thickness of 33 nm, in order to minimize the
reflectivity of the source/drain areas, and the buffer layer 24 is made of silicon
oxide. In this case, with an appropriate thickness of the buffer layer 24 (in particular
equal to 56 nm), it is possible to increase the reflectivity of the dielectric mirror
10, in particular by approximately 3% (from 94.5% to 97.3%), at the wavelength of
308 nm and for the three-pair configuration. This occurs in so far as the pair constituted
by the buffer layer and the capping layer acts as a further pair of the dielectric
mirror 10, even though the optical thicknesses of the layers are not exactly equal
to λ
0/4. It is to be moreover noted that this apparently small percentage increase in the
reflectivity of the dielectric mirror leads to an appreciable percentage decrease
(50%) in the transmitted radiation.
[0021] Next, an ELA treatment is carried out, for example with an XeCl excimer laser with
a wavelength λ
0 of 308 nm and an energy density of between 700 mJ/cm
2 and 1200 mJ/cm
2, for carrying out diffusion and annealing of the source/drain regions 30 previously
implanted. In particular, during the irradiation, the dielectric mirror 10 reflects
a fair share of the incident radiation, protecting the underlying gate structures
26, and an underlying second surface portion 20b of the layer of semiconductor material
20 (adjacent to, and distinct from, the first surface portion 20a), in this way preventing
their deformation. It is thus possible to use a high irradiation energy, and obtain
a complete removal of the crystallographic defects. Next, the capping layer 32 can
be removed, in the case where it comprises conductive material.
[0022] Subsequently (Figure 9), on the structures previously defined, a dielectric layer
(or dielectric multilayer) is formed, for example made of silicon oxide, silicon nitride,
or a combination of silicon nitride and silicon oxide. This dielectric layer is etched
in an anisotropic way so as to provide first spacers 35 at the sides of the gate structures
26 and of the dielectric mirror 10. The manufacturing process proceeds then with the
formation of the source/drain contacts of the MOS device.
[0023] In this regard (Figure 10), according to a first currently preferred variant of the
process, the implantation area of the source/drain regions 30 is previously defined
via an appropriate masking in such a way that the source/drain regions 30 have a lateral
extension smaller than the entire lateral extension of the body wells 28. Then, a
further etch of the layers still present on the source/drain regions 30 is carried
out, namely, of the capping layer 32 and of the gate-oxide layer 21, and a cleaning
of the source/drain contact is performed. Finally, the contact thus defined is filled
with one or more metal contact layers 37 so as to form the electrical source/drain
contact of the MOS device and moreover make a short circuit between body and source.
[0024] According to an alternative variant of the process, Figure 11, the source/drain contact
is obtained not by means of a photomasking process, but by resorting to digging of
a trench in the layer of semiconductor material 20 (a technique known as "micro-trench
technique" or "shallow-trench technique"). In particular, after the formation of the
first spacers 35, a further chemical etch of the layers still present on the source/drain
regions 30 is carried out (i.e., the capping layer 32 and the gate-oxide layer 21),
and also of part of the silicon of the layer of semiconductor material 20, using the
first spacers 35 as hard-mask. The etch is calibrated in order to dig a trench 38
having a depth greater than the depth of the source/drain regions 30. The metal contact
layer 37 fills the trench 28, in this way short-circuiting body and source.
[0025] In either case, the final structure is obtained (except for subsequent known finishing
steps) of the MOS device 40 illustrated in Figure 10 or Figure 11, in which the dielectric
mirror 10 performs an active function of electrical insulation between the gate electrodes,
constituted by the regions of the polysilicon layer 22 that have remained after the
etches described, and the source/drain electrodes, constituted by the metal contact
layer 37. Said structure can be advantageously used in all planar MOS structures,
whether they are vertical-conduction ones (for example, power MOSs, IGBTs, MCTs) or
lateral-conduction ones (NMOSs, PMOSs, CMOSs).
[0026] A second embodiment of the present invention envisages a different use and a different
arrangement of the dielectric mirror 10 during the manufacturing process of the MOS
device. Anyway, as will emerge clearly from what follows, also in this case, the dielectric
mirror 10 has the function of reflecting a fair share of the laser radiation incident
on the structure of the MOS device during the step of laser annealing so as to prevent
deformations of the structure and not jeopardize the electrical characteristics of
the MOS device.
[0027] In detail (Figures 12a-12b) after the definition of the active areas, an end point
layer 42 is formed (via growth or deposition) on the layer 20; the end point layer
42 is either single-layer (Figure 12a) or multilayer (Figure 12b), and is made up,
in this latter case, of a first end point layer 42a made of silicon oxide, having
a thickness of 5-50 nm, and a second end point layer 42b made of polysilicon, having
a thickness of 10-100 nm. Then, as described previously, the dielectric mirror 10
is formed, in particular, of the Bragg-reflector type, on the end point layer 42.
In particular, in the specific case of a dielectric mirror 10 made with alternating
layers of silicon oxide and silicon nitride, the end point layer 42 is advantageously
made up of the first end point layer 42a and the second end point layer 42b. Then,
a buffer layer 24 is deposited, having an optical thickness equal to n·λ
0/2-n
Cd
c, where n
c and d
c are, respectively, the refractive index and the thickness of a capping layer that
will be subsequently formed (in a way similar to what has been described previously).
Next, a masking and a selective anisotropic etch of the buffer layer 24 and of the
layers that make up the dielectric mirror 10 are carried out, said etch stopping on
the end point layer 42.
[0028] A dielectric layer (for example, made of silicon oxide or silicon nitride) is then
deposited, and subsequently etched (Figure 13) for the formation of second spacers
43 in a position adjacent to the side walls of the dielectric mirror 10. In particular,
for reasons that will emerge clearly from what follows, the thickness of the deposited
dielectric layer must be such that the width at the base of the second spacers 43
(designated by L
s in Figure 13) is greater than or equal to an extension (designated by L
b) of a lateral diffusion of a body well 28 that will be formed in a subsequent process
step (and that for said reason is illustrated with a dashed line in Figure 13). Furthermore,
if necessary for carrying out subsequent ion implants, the end point layer 42 can
be etched, where it is not covered by the second spacers 43 (as illustrated in Figure
13).
[0029] Then (Figure 14), within the first surface portion 20a of the layer of semiconductor
material 20, a first ion implantation is performed, for example of a P type with boron
(B) atoms, with a dose of 10
13-10
14 atoms/cm
2 and an energy of between 20 keV and 150 keV, in order to form the body wells 28 in
a way self-aligned to the second spacers 43. As mentioned previously, the body wells
28 diffuse laterally partially underneath the second spacers 43 for an extension L
b that is smaller than the width at the base L
s of the second spacers 43 in such a way as not to reach the region underlying the
dielectric mirror 10 (second surface portion 20b). The previously implanted dopant
atoms are then activated with a conventional thermal process, for example of the duration
of 30 minutes at a temperature of 900-1050°C. After possibly defining, via an appropriate
masking, the regions of the layer of semiconductor material 20 that are to undergo
the source implant, a second ion implantation is performed, at a high dose, in the
example in question of an N type with atoms of arsenic (As) or phosphorus (P) at a
dose of 10
14-10
15 atoms/cm
2 and with an energy of between 20 keV and 80 keV, in order to form source regions
45 within the body wells 28 at the sides of the second spacers 43. Next, on the structures
previously formed, a conformable capping layer 32 is deposited, which has the function
of improving the energy coupling with the laser beam in the subsequent annealing step.
As described previously, the presence of said capping layer must not reduce the reflectivity
of the dielectric mirror. An ELA treatment is then carried out, for example with an
XeCl excimer laser with a wavelength λ of 308 nm and an energy density of between
700 mJ/cm
2 and 1200 mJ/cm
2, for carrying out diffusion and annealing of the previously implanted source regions
45. In particular, during said irradiation step, the dielectric mirror 10 reflects
much of the incident radiation so as to prevent deformation of the underlying structures
of the MOS device and, in particular, of the end point layer 42 and the second surface
portion 20b of the layer of semiconductor material 20. Furthermore, a third surface
portion 20c of the layer of semiconductor material 20, in which the channel area of
the MOS devices is situated, underlying the second spacers 43, is shielded from the
laser radiation by the presence of the polysilicon in the end point layer 42 (in particular,
in the second end point layer 42b). Possible deformations of the end point layer 42
induced by the laser irradiation are limited to the areas not covered by the dielectric
mirror 10 and are in any case of no influence, in so far as the end point layer above
the third surface portion 20c is removed in the subsequent process steps. Furthermore,
the dielectric mirror 10, by blocking the incident laser radiation, causes the annealing
of the implanted regions (entailing the melting and subsequent re-solidification of
the semiconductor material) to be effectively limited to the areas of interest (source
regions 45) and not to spread to the portions of the layer of semiconductor material
20 situated underneath the dielectric mirror 10 (with the risk of jeopardizing operation
of the MOS device). Once again, given the high energy of the laser radiation, it is
possible to obtain a complete removal of the crystallographic defects due to the preceding
ion implantation. Next, the capping layer 32 can be removed.
[0030] Then (Figure 15), an isotropic etch of the second spacers 43 and of the end point
layer 42 underlying the second spacers 43 is carried out, and a gate-oxide layer 47
is formed (by either growth or deposition) in a position adjacent to the sides of
the dielectric mirror 10. According to an alternative embodiment, in the case where
the end point layer 42 is a multilayer, the etch can concern only the second end point
layer 42b of polysilicon that may have been damaged during the step of laser irradiation,
and the first end point layer 42a can be used as gate oxide of the MOS device. In
this case, the manufacturing process may not contemplate the dedicated step of formation
of the gate-oxide layer 47. In any case, a polysilicon layer 48, possibly doped, is
then formed (for example, by deposition) for making the gate electrodes of the MOS
device and thus forming the gate structures 26.
[0031] At this point, a step of photomasking to define the geometries of the polysilicon
and a step of selective anisotropic etching of the polysilicon layer 48 are performed.
According to the structure of a resist mask 49 (Figures 16-18) used for the purpose,
it is possible to obtain three different final structures (except for further finishing
steps) of the MOS device 40.
[0032] According to a first variant (Figure 16), the resist mask 49 is not present in the
area in which the active area of the MOS device is formed, but only on the service
areas of the edge termination (not illustrated) and of the gate contact pads (which
are not illustrated either). The anisotropic etch of the polysilicon layer 48 thus
determines the formation of first gate portions 50a at the sides of the dielectric
mirror 10, which act as gate electrodes of the MOS device. In particular, the width
at the base of said first gate portions 50a, designated by L
p, must be greater than or equal to the width at the base L
s of the second spacers 43, in such a way that the gate electrodes overlie a portion
of a respective source region 45. The thickness of the polysilicon layer 48 is then
constrained to the dimensions of the implant spacers and thus of the lateral diffusion
of the body wells. Furthermore, the dielectric mirror 10 insulates the first adjacent
gate portions 50a from one another.
[0033] In accordance with a second variant (Figure 17), the resist mask 49 is present in
the active area and coats a portion of the polysilicon layer 48 inside the area superimposed
on the dielectric mirror 10 (the resist mask 49 is illustrated with a dashed line
in Figure 17 so as to highlight the regions involved in the etch). In this case, following
upon the selective etch, further second gate portions 50b are formed on the dielectric
mirror 10 (in addition to the first gate portions 50a), and a further conductive path
of the polysilicon is thus created, which can be appropriately used for reducing the
internal gate resistance of the MOS structure (in a known way which is described,
for example, in the
Italian patent application No. MI2004A002244, filed on November 19, 2004). Also in this case, the thickness of the polysilicon
layer 48 is constrained by the lateral diffusion of the body wells 28.
[0034] According to a third variant (Figure 18), the resist mask 49 coats the entire dielectric
mirror 10 and extends until part of the source regions 45 is coated, so that, following
upon etching, a single gate portion 50c is formed above the dielectric mirror 10 and
surrounding the dielectric mirror 10 laterally. This solution enables the thickness
of the polysilicon layer 48 to be independent of the lateral diffusion of the body
wells, but, on the other hand, calls for a perfect alignment between the resist mask
49 and the mask that has defined the dielectric mirror 10. Furthermore, said solution,
in the case where the polysilicon layer 48 is coated with a further dielectric layer
of appropriate thickness (in a way not illustrated), lends itself to the production
of self-aligned contacts (in a known way which is described, for example, in
US 5,981,343).
[0035] A third embodiment of the present invention envisages a use of the dielectric mirror
10 similar to that of the second embodiment, but with some variants in the processes
of definition of the body wells 28 and of the source regions 45.
[0036] In detail (Figure 19), after definition of the active areas, the single-layer or
multilayer end point layer 42 is formed on the layer 20. As described previously,
on the end point layer 42 a temporary dielectric mirror 10' (the reason for said definition
will emerge clearly from what follows), and the buffer layer 24 having characteristics
such as not to alter the reflectivity of the temporary dielectric mirror 10' are formed.
Next, a masking and a selective anisotropic etch of the buffer layer 24 and of the
layers that make up the temporary dielectric mirror 10' are carried out, with the
etch arresting at the end point layer 42. The end point layer 42 is then removed from
the areas not masked by the dielectric mirror.
[0037] Then (Figure 20), at the first surface portion 20a of the layer of semiconductor
material 20, a first ion implantation is performed, for example of a P type with boron
(B) atoms at a dose of 10
13-10
14 atoms/cm
2 and with an energy of between 20 keV and 150 keV in order to form the body wells
28 in a way self-aligned to the temporary dielectric mirror 10'. The previously implanted
dopant atoms are then activated with a conventional thermal process, for example having
the duration of 30 minutes at a temperature of 900-1050°C. During the annealing process,
the body wells 28 diffuse laterally partially underneath the temporary dielectric
mirror 10' for an extension L
b (defining the second surface portion 20b underneath the temporary dielectric mirror
10', as the area not involved in said diffusion). After possible definition, via an
appropriate masking, of the regions of the layer of semiconductor material 20 that
are to undergo the source implant, a second ion implantation is performed at a high
dosage, in the example of an N type with atoms of arsenic (As) or phosphorus (P) at
a dose of 10
14-10
15 atoms/cm
2 and with an energy of between 20 keV and 80 keV, in order to form the source regions
45 within the body wells.
[0038] Next, a conformable capping layer 32 is deposited on the structures previously formed
(Figure 21); the conformable capping layer 32 has the purpose of minimizing the reflectivity
of the surface of the source/drain areas at the wavelength that will be used for irradiation
and of leaving the reflectivity of the temporary dielectric mirror 10' unaltered,
the combined optical thickness of the buffer and capping layers being equal to n·λ
0/2. An ELA treatment is then carried out, for example with an XeCl excimer laser at
a wavelength λ of 308 nm and with an energy density of between 700 mJ/cm
2 and 1200 mJ/cm
2, for carrying out the diffusion and the annealing of the previously implanted source
regions 45. During said irradiation step, the temporary dielectric mirror 10' reflects
a fair share of the incident radiation so as to prevent deformation of the underlying
structures of the MOS device, and in particular of the end point layer 42, of the
second surface portion 20b of the layer of semiconductor material 20, and of the channel
region defined by the lateral diffusion of the body well 28 (third portion 20c of
the layer of semiconductor material 20). Next, the capping layer 32 is removed.
[0039] Then (Figure 22), an isotropic etch of the buffer layer 24 and of the layers that
make up the temporary dielectric mirror 10' is carried out so as to reduce their dimensions
and form the dielectric mirror 10 (which will remain in the final structure). Said
etch can be carried out using just one solution, etching the various layers simultaneously,
or by means of successive etches with solutions dedicated to the various each materials.
In particular, the duration of the etch is calibrated so as to reduce a main dimension
of the temporary dielectric mirror 10' (in particular, a dimension in a direction
parallel to the surface of the layer of semiconductor material 20) by a distance L
e (for each side) greater than or equal to the extension L
b of the lateral diffusion of the body well 28. Then, it is possible to eliminate,
if necessary, the buffer layer 24.
[0040] Next, the gate-oxide layer 47 is formed, via growth or deposition (Figure 23) in
an adjacent position at the sides of the dielectric mirror 10. As described previously,
in the case where the end point layer 42 is a multilayer, the etch can concern only
the second end point layer 42b of polysilicon, which may have been damaged during
the step of laser irradiation, and the first end point layer 42a can be used as gate
oxide of the MOS device. In this case, the manufacturing process may not contemplate
the step of dedicated formation of the gate-oxide layer 47. In any case, a polysilicon
layer 48, possibly doped, is then formed (for example by deposition), for making the
gate electrodes of the MOS device, and the gate structures 26 are formed in a way
similar to what was described previously in relation to the second embodiment.
[0041] In all of the solutions described above, in the resulting MOS device 40 the dielectric
mirror 10 has the function of insulating the gate electrodes and the drain electrode
of the MOS device and the function of intercell thick dielectric between adjacent
gate structures, with the advantage of reducing the capacitances between the gate
and drain electrodes of the MOS device. Furthermore, the second and third embodiments
described can advantageously be used in all vertical-conduction planar MOS structures
(for example, power MOSs, IGBTs, MCTs). Said embodiments can be used also in the case
where the starting layer of semiconductor material is constituted by a semiconductor
material other than silicon, for example silicon carbide (SiC). In this regard, as
is known, for activation and diffusion of the dopants in the silicon carbide, temperatures
higher than the melting point of silicon are necessary, so that it is extremely advantageous
to form the gate structures (or part of the gate structures in the case where the
end point layer 42 is used in part as gate oxide) subsequent to the steps of activation
and annealing of the body and source regions.
[0042] The advantages of the described manufacturing process emerge clearly from the foregoing
description.
[0043] In any case, it is emphasized that the use of the dielectric mirror 10 in the process
for manufacturing the MOS device 40 enables protection of the structures previously
formed by the laser irradiation for the annealing of the doped source/drain regions.
In this way, it is possible to use high energies of irradiation (even higher than
1 J/cm
2), so as to obtain a complete elimination of the implantation defects and at the same
time prevent damage to the structure of the MOS device, in particular, to the gate
structures.
[0044] Furthermore, as described, the presence of the dielectric mirror 10 as element of
insulation in the final MOS device, is advantageous in all the embodiments described.
In particular, in the first embodiment, the dielectric mirror enables insulation between
the gate electrodes and the source/drain contacts to be obtained, whilst in the second
and third embodiments, the dielectric mirror enables a significant reduction in the
capacitances of the gate electrodes, acting as thick intercell dielectric.
[0045] Advantageously, the dielectric mirror 10 can be used for obtaining both vertical-conduction
and lateral-conduction MOS devices (in accordance with the respective process variants).
[0046] Finally, it is clear that modifications and variations can be made to what is described
and illustrated herein, without thereby departing from the scope of the present invention,
as defined in the annexed claims.
[0047] In particular, the dopant species used for the implants can be reversed for producing
P-channel, instead of N-channel, devices.
[0048] Furthermore, a different type of excimer laser can be used for the process of annealing,
for example an XeF laser (at a wavelength λ of 351 nm), KrF (λ = 248 nm), or ArF (λ
= 193 nm), or other types of laser at a wavelength lying in the spectrum of the ultraviolet.
Clearly, by varying the laser used, the thickness of the layers that constitute the
dielectric mirror will consequently vary, according to the laws previously described.
[0049] Finally, the layer of semiconductor material 20, within which the active areas of
the MOS device are formed, can be an epitaxial layer grown on top of a substrate,
the substrate and the epitaxial layer possibly having the same type of conductivity
and different doping concentrations, or possibly different types of conductivity.
1. A process for manufacturing a MOS device (40), comprising:
- providing a semiconductor layer (20);
- forming a gate structure (26) above said semiconductor layer (20) ;
- forming a first doped region (30; 45) within a first surface portion (20a) of said
semiconductor layer (20); and
- irradiating said first doped region (30; 45) with electromagnetic radiation to carry
out annealing thereof;
characterized by forming a dielectric mirror (10) above a second surface portion (20b) of said semiconductor
layer (20), prior to said irradiating step, said dielectric mirror (10) being configured
to reflect at least in part said electromagnetic radiation.
2. The process according to claim 1, wherein forming a dielectric mirror (10) comprises:
forming a plurality of pairs of a first mirror layer (10a) and a second mirror layer
(10b), set on top of one another, said first mirror layer (10a) and said second mirror
layer (10b) having, respectively, a first refractive index (nh) and a second refractive index (nl) that are different from one another; and etching said first mirror layer (10a) and
said second mirror layer (10b) so as to form said dielectric mirror (10).
3. The process according to claim 2, wherein said dielectric mirror (10) is a Bragg reflector,
and said electromagnetic radiation has a given wavelength (λ0) ; a first optical thickness of said first mirror layer (10a) and of said second
mirror layer (10b) being equal to a multiple (n) of a quarter of said given wavelength
(λ0).
4. The process according to any one of the preceding claims, further comprising, prior
to said irradiating step, forming a buffer region (24) on said dielectric mirror (10),
said buffer region (24) having optical characteristics such as not to reduce a reflectivity
of said dielectric mirror (10).
5. The process according to claim 4, wherein said electromagnetic radiation has a given
wavelength (λ0); further comprising, prior to said irradiating step, forming a capping region (32)
on said buffer layer (24) and above said first surface portion (20a) of said semiconductor
layer (20), said capping region (32) having optical characteristics such as to improve
an energy coupling with said electromagnetic radiation, and such as not to reduce
said reflectivity of said dielectric mirror (10); in particular, said capping region
(32) comprising a dielectric material and having a second optical thickness (nc·dc), and said buffer region (24) having a third optical thickness given by the difference
between a multiple of one half of said given wavelength (λ0), and said second optical thickness (nc·dc).
6. The process according to any one of the preceding claims, wherein forming a dielectric
mirror (10) comprises forming said dielectric mirror (10) above said gate structure
(26), said dielectric mirror (10) being configured to shield said gate structure (26)
from said electromagnetic radiation.
7. The process according to claim 6, wherein forming a first doped region (30) comprises
carrying out an introduction of dopant species in a way aligned with respect to said
gate structure (26) and to said dielectric mirror (10), in order to form said first
doped region (30) in a position adjacent to, and partially underneath, said gate structure.
8. The process according to claim 6 or 7, further comprising forming a first conductive
region (23) set between said gate structure (26) and said dielectric mirror (10);
said first conductive region (23) comprising, in particular, a metal or a metallic
silicide, chosen preferably in the group comprising: W, Mo, Ti, Ta, CoSix, PtSix, TiSix, WSix.
9. The process according to any one of claims 6-8, further comprising, prior to said
irradiating step, forming a capping region (32) on said dielectric mirror (10); and
wherein forming a gate structure (26) comprises:
- forming a first gate dielectric layer (21) on said semiconductor layer (20);
- forming a first gate conductive layer (22) on said first gate dielectric layer (21);
and
- selectively removing portions of said first gate conductive layer, thereby forming
a first gate electrode (22);
further comprising oxidizing side walls of said first gate electrode (22) prior to
forming said capping region (32).
10. The process according to any one of claims 6-9, wherein forming a dielectric mirror
(10) comprises: forming a plurality of pairs of a first mirror layer (10a) and a second
mirror layer (10b), set on top of one another, at least one between said first mirror
layer (10a) and said second mirror layer (10b) comprising an insulating material;
further comprising:
- forming a second doped region (28) within said semiconductor layer (20), at the
sides of, and partially underneath, said gate structure (26), forming a first doped
region comprising forming said first doped region (30) within said second doped region
(28);
- forming a conductive layer (37) above said dielectric mirror (10) and said semiconductor
layer (20), said conductive layer configured to contact said first doped region (30)
and said second doped region (28).
11. The process according to claim 10, further comprising forming first spacers (35) at
the sides of said gate structure (26) and said dielectric mirror (10) prior to forming
said conductive layer (37); and wherein forming a first doped region (30) comprises
defining a lateral extension of said first doped region (30), said lateral extension
being smaller than a main dimension of said second doped region (28).
12. The process according to claim 10, further comprising forming first spacers (35) at
the sides of said gate structure (26) and of said dielectric mirror (10) prior to
forming said conductive layer (37); and etching said semiconductor layer (20) to dig
a trench (38) having a depth greater than a depth of said first doped region (30),
in such a manner as to reach said second doped region (28); forming a second conductive
layer (37) comprising filling said trench (38).
13. The process according to any one of claims 1-5, wherein forming a dielectric mirror
(10) comprises forming said dielectric mirror at said second surface portion (20b)
of said semiconductor layer (20), said dielectric mirror (10) being configured to
protect said second surface portion (20b) from said electromagnetic radiation; and
wherein the step of forming a gate structure (26) is performed subsequent to said
irradiating step.
14. The process according to claim 13, wherein forming a first doped region (45) comprises
forming said first doped region in a position at a lateral distance from said dielectric
mirror (10).
15. The process according to claim 14, wherein forming a dielectric mirror (10) comprises
forming a plurality of pairs of a first mirror layer (10a) and a second mirror layer
(10b), set on top of one another, at least one between said first mirror layer (10a)
and said second mirror layer (10b) comprising an insulating material; and wherein
forming a gate structure (26) comprises:
- forming a second gate dielectric layer (47) on said semiconductor layer (20), in
a position adjacent to said dielectric mirror (10);
- forming a second gate conductive layer (48) on said second gate dielectric layer
(47), and above, and surrounding, said dielectric mirror (10); and
- selectively removing portions of said second gate conductive layer, thereby forming
a second gate electrode (50a-50c).
16. The process according to claim 15, further comprising, prior to forming said dielectric
mirror (10), forming an end point layer (42a), including dielectric material, on said
semiconductor layer (20); and, prior to forming a gate structure (26), removing said
end point layer (42a) from the areas not covered by said dielectric mirror.
17. The process according to claim 15 or claim 16, wherein selectively removing comprises
one between:
- selectively removing portions of said second gate conductive layer (48) in such
a manner as to leave first gate-electrode portions (50a) laterally in contact with
said dielectric mirror (10), said first gate-electrode portions (50a) having a second
thickness at the base (Lp) not less than said first thickness at the base (Ls) of said second spacers (43); or
- selectively removing portions of said second gate conductive layer (48) in such
a manner as to leave, in addition to said first gate-electrode portions (50a), second
gate-electrode portions (50b) arranged on said dielectric mirror (10); or
- selectively removing portions of said second gate conductive layer (48) so as to
leave a single gate portion (50c) arranged laterally in contact with, and on, said
dielectric mirror (10), and extending in part above said first doped region (45) and
said second doped region (28).
18. The process according to any one of claims 14-17, wherein forming said first doped
region (45) in a position at a lateral distance from said dielectric mirror (10) comprises:
- forming second spacers (43) at the sides of said dielectric mirror (10);
- forming a second doped region (28) within said semiconductor layer (20), at the
sides of, and partially underneath, said second spacers (43);
- carrying out a thermal treatment of said second doped region (28), which causes
a lateral diffusion of said second doped region (28) for a lateral extension (Lb), said second spacers (43) having a first thickness at the base (Ls) greater than said lateral extension (Lb); and
- carrying out an introduction of dopant species in a way aligned with respect to
said second spacers (43) so as to form said first doped region (45) within said second
doped region (28).
19. The process according to any one of claims 14-17, wherein forming a dielectric mirror
(10) comprises forming a temporary dielectric mirror (10') in an area corresponding
to said second surface portion (20b) of said semiconductor layer (20), and reducing
said temporary dielectric mirror (10') so as to form said dielectric mirror (10);
and wherein forming said doped region (45) in a position at a lateral distance from
said dielectric mirror (10) comprises:
- forming a second doped region (28) within said semiconductor layer (20), at the
sides of, and partially underneath, said temporary dielectric mirror (10');
- carrying out a thermal treatment of said second doped region (28), which causes
a lateral diffusion of said second doped region (28) for a lateral extension (Lb); and
- carrying out an introduction of dopant species in a way aligned with respect to
said temporary dielectric mirror (10') to form said first doped region (45) within
said second doped region (28),
- reducing said temporary dielectric mirror (10') comprising reducing a main dimension
of said temporary dielectric mirror (10') by an amount (Le) of not less than said lateral extension (Lb).
20. A MOS device (40) comprising:
- a semiconductor body (20);
- a gate structure (26) arranged above said semiconductor body (20) and including
a gate electrode (22; 50a-50c); and
- a first doped region (30; 45), formed within a first surface portion (20a) of said
semiconductor body (20), at the sides of and partially underneath said gate structure,
characterized by comprising a dielectric mirror (10) arranged above a second surface portion (20b)
of said semiconductor body (20).
21. The device according to claim 20, wherein said dielectric mirror (10) is insulating
and is arranged in a position adjacent to said gate electrode (22; 50a-50c) so as
to insulate said gate electrode (22; 50a-50c).
22. The device according to claim 20 or 21, wherein said dielectric mirror (10) is a Bragg
reflector and comprises a plurality of pairs of a first mirror layer (10a) and a second
mirror layer (10b), set on top of one another; said first mirror layer (10a) and said
second mirror layer (10b) having, respectively, a first refractive index and a second
refractive index (nh, nl), which are different from one another.
23. The device according to any one of claims 20-22, further comprising a buffer region
(24) on said dielectric mirror (10), said buffer region (24) having optical characteristics
such as not to reduce a reflectivity of said dielectric mirror (10).
24. The device according to any one of claims 20-23, further comprising a capping region
(32) on said buffer layer (24) and above said first surface portion (20a) of said
semiconductor layer (20), said capping region (32) having optical characteristics
such as to improve an energy coupling with an incident electromagnetic radiation,
and not to reduce a reflectivity of said dielectric mirror (10).
25. The device according to any one of claims 20-24, wherein said dielectric mirror (10)
is set above said gate electrode (22); further comprising a first electrical-contact
structure (37), in electrical contact with said first doped region (30) and set above
said dielectric mirror (10).
26. The device according to claim 25, further comprising a first conductive region (23)
set between said gate electrode (22) and said dielectric mirror (10); said first conductive
region (23) comprising, in particular, a metal or a metallic silicide, chosen preferably
in the group comprising: W, Mo, Ti, Ta, CoSix, PtSix, TiSix, WSix.
27. The device according to claim 25 or claim 26, wherein said semiconductor body (20)
has a first type of conductivity, further comprising:
- a second doped region (28) having a second type of conductivity set within said
semiconductor body (20), at the sides of, and partially underneath, said gate structure
(26), said first doped region (30) being set within said second doped region (28),
and having said first type of conductivity; and
- first spacers (35), made of insulating material, set at the sides of said gate structure
(26) and of said dielectric mirror (10),
said first electrical-contact structure (37) being in electrical contact also with
said second doped region (28).
28. The device according to claim 27, in which said first doped region (30) has a lateral
extension smaller than a principal dimension of said second doped region (28), and
said first electrical-contact structure (37) is set above, and directly in contact
with, said first doped region (30) and said second doped region (28).
29. The device according to claim 27, further comprising a cavity (38) made within said
semiconductor body (20) and having a depth greater than a depth of said first doped
region (30) and such as to reach said second doped region (28); said first electrical-contact
structure (37) being set partially within said cavity (38).
30. The device according to any one of claims 20-24, wherein said dielectric mirror (10)
is set in an area corresponding to said semiconductor layer (20), and said gate electrode
(50a-50c) is set at least in part above, and in contact with, said dielectric mirror
(10).
31. The device according to claim 30, wherein said gate structure (26) further comprises
a gate dielectric layer (47) set on said semiconductor layer (20), laterally in contact
with said dielectric mirror (10).
32. The device according to claim 30, further comprising an end point layer (42a) on said
semiconductor layer (20), comprising dielectric material, said dielectric mirror (10)
being arranged on said end point layer (42a); and wherein said gate structure (26)
further comprises part of said end point layer (42a).
33. The device according to any one of claims 30-32, wherein said semiconductor body (20)
has a first type of conductivity, further comprising a second doped region (28) having
a second type of conductivity arranged within said semiconductor body (20), at the
sides of, and partially underneath, said gate structure (26), said first doped region
(45) being set within said second doped region (28) and having said first type of
conductivity; said gate electrode (48) comprising one between:
- first gate portions (50a) set laterally in contact with said dielectric mirror (10),
and extending above said first and second doped regions (45, 28); or
- in addition to said first gate portions (50a), second gate portions (50b) set on
said dielectric mirror (10); or
- a single gate portion (50c) set laterally in contact with, and above, said dielectric
mirror (10), and extending above said first doped region (45) and said second doped
region (28).
34. The device according to any one of claims 20-33, wherein said semiconductor body (20)
comprises one between: silicon (Si) and silicon carbide (SiC).